This invention relates to apparatus for driving magnetic recording and reproducing apparatus required to selectively switch the running speed of magnetic recording medium or tape over a wide range, more particularly to apparatus which utilizes an intermittently rotating pulse motor for driving magnetic recording and reproducing apparatus which is required to be operated smoothly at a constant speed over a range covering low speeds that can not be provided by an ordinary induction motor or a synchronous motor and high speeds.
In magnetic recording and reproducing apparatus (for the sake of simplicify, hereinafter merely termed a magnetic recorder) various methods and apparatus have been used to drive the magnetic tape at a stable and constant speed for performing recording and reproduction at high fidelities. Accordingly, the driving motor for driving a capstan that determines the running speed of the tape is required to rotate stably at a constant speed. For this reason, in most cases synchronous motors are preferred rather than induction motors. Where synchronous motors are used, switching of the running speed of the tape is effected by changing the number of poles of the synchronous motor or by using a plurality of intermediate idle rollers which are switched mechanically. However, where it is necessary to switch the running speed of the tape among several different speeds, the number of poles and the number of intermediate idle rollers increase also. For example, where it is necessary to select one of four different speeds, it is necessary to change the number poles among, 2, 4, 8 and 16 poles, thus increasing size of the motor and complicating its connection. In the case of switching among idle rollers of different diameter, the mechanism for sliding and selecting idle rollers becomes complicated. Where these two types of speed switching is used, the weight of the speed changing mechanism increases. In addition, it is necessary to construct the mechanism to be adjustable and it has been difficult to obtain absolutely constant speeds due to the deformation of the idle rollers or the like causes.
Where it is desired to vary the running speed of the tape over a wide range of from an extremely low speed to a high speed, for example 100 mm/sec., an ordinary induction motor or a synchronous motor and a mechanical speed changing mechanism can not meet this requirement. In an example of a video tape recorder which operates the tapes for 1 hour, 6 hours, 12 hours, 24 hours and 48 hours, respectively, it is necessary to change the speed of the tape for respective runs. Such long time video tape recorder is used in combination with a television camera for the supervision over a long period, recording and observation of various human activities. For example, such a system has a wide application in the long time supervision of the safeness of banks or the like place, for continuous supervision of nuclear reactors to which access of the operators is dangerous, for observation of the condition of department stores or stations where many persons gather together or an analysis of the styles or physical reactions of sportmen. Such a video tape recorder usually uses a video tape of the 3/4 inch cassette type, and in addition to a 48 minutes recording, by operating a transfer switch it is also possible to record for 6, 12, 24 or 48 hours.
In such a long time video tape recorder, the running speed of the tape is the highest for the one hour mode but decreases for operating modes over longer time periods. For this reason, taking the one hour mode as the standard operation, if the recording is made for 6 hours and the reproduction is made for 12 hours, a slow motion picture would result. On the other hand, an extremely slow motion, such as the procedure of the blooming of a flower, is recorded for 48 hours and the recorded film is reproduced by the one hour mode. Then the procedure of the blooming can be reproduced as a continuous motion. In such a case, it is often desired to stop the tape to reproduce a standstill picture. As has been described hereinabove, the running speed of the tape of such a video tape recorder is required to vary between the shortest time mode and the longest time mode, the ratio being 1:48 for example. For example, the minimum running speed of the tape for the one hour mode operation is about 118.8 mm/sec., whereas those for the 6, 12, 24 and 48 hour mode operations are about 14.8, 7.4, 3.7 and 1.8 mm/sec., respectively.